Adult brownbanded bamboo sharks may still have faint bands (ZulRosle/Flickr)

Before they even leave their eggs, embryonic bamboo sharks freeze in response to perceived danger – a behavior that may help scientists develop a more effective shark repellent.

The still-developing bamboo sharks are responding to electrical fields generated by predators, a study published today in PLoS ONE reports.

Bamboo sharks live in shallow, west Pacific waters and can grow up to 3.5 feet long. Unlike some other shark species, they don’t give birth to live young. Instead, embryos develop in eggs, taking as long as five months to reach hatching age. During that time, the egg-enclosed sharks are vulnerable to predation by large fishes and other sharks, which can sense the odors, water flow, and electric field produced by thrashing tails and moving gills.

“A lot of people tend to think of sharks as predators instead of the prey,” said Stephen Kajiura, a marine biologist at Florida Atlantic University, who was not involved in the work. “Here you have a good demonstration of how little baby sharks who get eaten by all sorts of things have to watch out for themselves, basically, at early developmental stages as well.”

To ask whether embryonic brownbanded bamboo sharks (Chiloscyllium punctatum) could sense danger, scientists first snagged eggs laid by captive animals in Australia. Removing the egg’s opaque covering allowed researchers to peer at the shark growing inside. By the time the sharks are ready to hatch, they’re between 4 and 5 inches long, said Ryan Kempster, a graduate student at The University of Western Australia.

When the team introduced an electric field mimicking the signal produced by approaching bamboo shark predators, the developing embryos ceased gill movements, wrapped their tails around their bodies, and held still – some, for nearly a minute. Freezing is a typical prey response to a predator when no alternative – such as fight or flight – is present, Kempster said.

When the baby sharks experienced the same electric field multiple times with no consequences, they began to freeze for shorter periods of time — meaning that they learned the field wasn’t dangerous.

Kempster suggests that such observations could be used to help develop non-lethal shark control measures. “Sharks may become conditioned to current repellent devices if the signals that these devices produce do not change substantially over time,” Kempster said. “More research funding should be invested into producing an effective shark repellent. Such devices will also be useful in reducing shark bycatch, by keeping sharks away from fishing gear, to decrease the number of sharks unnecessarily killed each year.”